Electrical power systems have long been recognized as having the potential to be used as an effective communication channel but in practice, this potential is severely restricted due to the strong power signal being transmitted and the harmonics of the power signal, as well as the frequency characteristics of local power distribution equipment. To overcome this problem, some powerline carrier line systems have used a high frequency communication signal which is not affected by the power signal. Unfortunately, such high frequency signals encounter problems where the power distribution system includes power factor correction capacitor banks. In order for the high frequency signal to be transmitted, these capacitor banks have to be trapped. Unfortunately, it is difficult and expensive to carry this out and it is even more difficult to ensure that this always occurs. Failure to take this corrective step results in the loss of the communication signal. The high cost and the difficulty of controlling this arrangement renders a high frequency transmitter system ineffective. The potential of powerline transmission systems being used as a communication channel continues to be attractive, however, the effective use of this potential communication path has proven difficult to realize.
Powerline carrier systems (PLC) can be divided into two segments, a powerline transmission segment and a downstream powerline distribution network segment.
PLC systems applied to distribution networks have a group of special technical obstacles that are not experienced in transmission level PLC. Distribution line carrier (DLC) signals must propagate through networks that are extremely hard to model as the networks branch and mesh with each other while experiencing highly variable levels of loading. DLC signals must traverse a network that was designed to carry power signals at 50/60 Hz and are optimized for this task. Power transformers at PLC frequencies are modelled primarily, at PLC frequencies, by their leakage reactance and tend to block DLC signals. Capacitor banks used for power factor correction present a low-impedance path to ground and sink DLC signals unless they are trapped out with reactors.
Finally, standing-wave phenomena cause many nodal points to occur throughout the distribution network when carrier frequencies become greater than 5 kHz. To obviate this problem, many carrier frequencies can be employed simultaneously but at the cost of increased receiver complexity.
These obstacles lead to the development of so-called ripple control system in the 1960s for use with residential and commercial load shedding systems that helped utilities offset peak demand and maintain service during period of generation shortfall. Traditional residential load-shedding systems controlled appliances by transmitting signals having very low information content and low bandwidths. Ripple control systems relied on binary on/off signalling, or amplitude shift keying (ASK), at typical signalling rates of between 0.5 and 5 baud. Messages were sent as broadcast "telegrams" to residential loads to either turn on or off consumption, typically with a safety time-out mechanism in case the load failed to receive a turn on signal after a load shed request. This was necessary as signals were commonly not received or not recognized. Carrier frequencies of ripple systems were kept very low, between 30-1000 Hz, in order to avoid the cost of distribution network changes but placed the signals in the most noisy area of the powerline spectrum. The magnitude of power frequency harmonics can be very large, with respect to the fundamental, below 2.5 kHz and jam any communication systems that use this frequency range to transmit.
Ripple systems with their low signal rates require only about 10 Hz of bandwidth to communicate and could easily fit between two 50/60 Hz harmonics. The problem during the 1960s and 1970s was that the narrow band filters used to isolate the ripple signal from the noise usually let in more power frequency harmonics than they did signal. The solution for many utilities, even today, is to drastically increase the DLC injected power to the point where the DLC signal becomes many times larger than the nearest harmonics. The amount of injected power is measured in kilowatts so most ripple systems use motor-generator pairs to inject the signal; a very costly solution.
Higher frequency DLC systems, above 5 kHz, were also available for utilities that required higher data throughput or two-way communications. Westinghouse, General Electric, and Rockwell all offered such systems during the 1980s. As mentioned before, all these higher frequency systems required distribution network changes to accommodate the DLC signals and most used multiple carrier frequencies to overcome standing-wave phenomena.
Today, only a few companies produce DLC systems, as the cost has become prohibitive for the typical applications of automatic meter reading (AMR) and load shedding. The plummeting cost of radio communications, the need for more bandwidth, and the proliferation of estimated consumption billing, has caused many utilities to abandon DLC systems. The original advantages of DLC systems still exist, if only the cost can be dramatically reduced, and preferrably the data-rate brought up to a level that would enable other revenue generating/customer attention services such as real-time pricing, residential information (weather, news etc.) and remote service disconnection.
For the above reasons, power carrier systems have not proven popular. The present application overcomes these difficulties and combines this form of communication path with electrical equipment to be placed in the home. The powerline carrier system is used to broadcast signals into the homes. These broadcast signals can be saved temporarily for review by the occupants of the home and the broadcast signals can include instruction signals for controlling certain devices in the home. The present invention also combines this powerline carrier broadcast system with a different communication channel out of the home or premise for reporting to a central source or predetermined computer.